In an effort to satisfy the public's desire for more comfortable lenses that can be worn for extended periods of time without removal, contact lens materials and designs are currently being pushed to the limits of physiological acceptance by the cornea. Good vision requires a stable lens shape, with minimal flexing or distortion of the lens; the provisions for comfort and safety, however, are more complex, involving maintenance of a wet lens surface and transfer of oxygen to the cornea. Traditional hard lenses must move during blinking to provide oxygen transfer, rigid and soft gas-permeable lenses need not move, and toric and bifocal designs must maintain a stable orientation and position on the eye. The prelens tear film provides a smooth refracting surface, helps wash away debris, lubricates lid motion, and prevents dehydration of hydrogel lens materials and adherence between lens, lids, and cornea. The true interactions between lenses and the lids and tear film must be determined under conditions of actual wear - in the blinking, tearing eye. A unique, powerful technique that utilizes high-speed imaging and sophisticated optical interference methods and allows exact determinations of tear film dynamics will be used to characterize these interactions. It literally provides a topographical map of the tear film thickness distribution and its changes over time (with 30 images/sec) on in vivo lens surfaces, as well as objective measurements of functional lens wettability. A high-speed camera records lens movement and flexure at rates of 500 pictures/sec, and subsequent frame-by-frame analysis provides detailed information. The use of this camera, a hi-resolution video system, and optical interference methods will provide an in vivo means to (1) quantitate spatial and temporal wetting, spreading, and thinning parameters, and the breakup times of the tear film on contact lens surfaces; (2) determine the translation movements of such lenses and correlate them with lens material, shape, and design; (3) measure the flexure and hysteresis/recovery characteristics of lenses, especially of "rigid" gas-permeable designs, to determine the limits of lens thickness and shape factors for these materials. The data obtained will aid in understanding how lenses perform in vivo and advance the development of improved lens designs and materials.